The process of migrating a liquid metal inclusion through a solid using a thermal gradient, known as Temperature Gradient Zone Melting (TGZM), was invented by Pfann and is described, for example, in his U.S. Pat. No. 2,813,048 issued Nov. 12, 1957. In Pfann's process either sheets or wires of a suitable metallic liquid were moved through a semiconductor material by a thermal gradient. Doped liquid-epitaxial material was left behind as the liquid wire migration progressed. Subsequently, several investigators found that the planar liquid zone was unstable and that droplets of liquid broke away from the zone and were trapped in the recrystallized material. For that reason, material produced by Pfann's TGZM process was not suitable for the manufacture of semiconductor devices.
The TGZM process has, however, recently been developed by Anthony and Cline into a viable commercial process for producing P-N junction structures in semiconductors such as, for example, the isolation grids described in their U.S. Pat. Nos. 3,979,230 and 3,998,662; the entirety of which are herein incorporated by reference. For detailed information on the recently developed TGZM process, apparatus, and P-N junction devices produced thereby, in general, reference may be made, for example, to U.S. Pat. Nos. 4,075,038; 4,091,257; and 4,224,594, respectively; the entirety of which are also herein incorporated by reference.
Briefly, in the commercial version of the TGZM process, semiconductor wafers are placed between an array of air-cooled high-temperature quartz lamps and a water-cooled heat sink. Infrared radiation from the lamps is absorbed and converted into heat by the wafer surface facing the lamps. This heat then passes through the wafer and is re-radiated to the heat sink by the opposing wafer surface. The thermal gradient that drives the TGZM process is generated by the heat flow through the wafer. Typical apparatus for the commercial practice of the TGZM process is described in U.S. Pat. No. 4,221,956; the entirety of which is incorporated herein by reference.